1. Field of the Invention
The present invention relates to an electrophotographic printing method and apparatus to be utilized in a copy machine, a printer in general, a facsimile and like and, more particularly, to an electrostatic toner transfer system of such an electrophotographic printing and associated features.
2. Description of the Background Art
There are presently two types of apparatus for the electrostatic transfer of toner image from photoconductive drum to receiving paper, one using corona charger, and the other using a conductive roller or a drum with externally applied voltage, which is described in U.S. Pat. No. 2,626,865.
Of these two, the one using the corona charger is popular for general monochromatic copy machines because of its simple structure. In this type of apparatus, electric charges are produced by the corona charger as corona ions generated by applying several kV voltage through a fine tangsten wire. The generated charges are then applied to the receiving paper from behind so that the toner is transferred from the photoconductive drum to the receiving paper by the electric fields due to the charges attached on the receiving papers.
It has been noted by the present inventors that in such an apparatus, the strength of the electric fields varies for different receiving papers as different resistivities of the different receiving papers changes the amounts of charges attached on the different receiving papers because of the charge leakage through the receiving paper, even for the same amount of charges generated by the corona charger. Such a difference in the electric field strength for different receiving papers affects the toner transfer efficiency.
Now, since papers usually used as receiving papers change their resistivities significantly, according to the surrounding humidity, and since the difference in the electric field strength for different receiving papers affects the toner transfer efficiency, it has been difficult to achieve consistent color printing, because the color balance in the color printing, in which different color toner are superposed, tends to be disturbed. Even in monochromatic printing, the fluctuation in image density due to the variation in humidity is common.
There is also a problem of image disturbances due to scattering of toner on the receiving papers caused by spark discharge from charges on the receiving papers to the photoconductive drum, occurring in contacting and detaching the receiving papers from the photoconductive drum. These are problematic enough for monochromatic printing, but are especially so for color printing where the color toners are required to be accurately superposed.
There have been attempts to cope with such problems. One proposition is to utilize an insulating mesh, as described in Japanese Patent Laid Open No. S56-164370. However, the toner transfer efficiency still varies as the resistivity of the receiving papers is changed by the surrounding humidity.
Another proposition is to utilize a soft foamed conductive rubber roller, as described in Japanese Patent Laid Open No. S50-22640. In this method, high quality image is obtainable and, in addition, the transfer to thick receiving papers such as envelops and those receiving papers with uneven soft surface is possible.
However, it has been difficult to manufacture a foamed conductive rubber roller in accurate shape. Moreover, in order to make the foamed rubber roller conductive, conductive particles such as conductive carbon black are mixed in, but the elasticity of the roller is changed by the amount of mixture so that the desired elasticity has been difficult to obtain. There is also a problem concerning the discharge inside of the foamed conductive rubber roller, which shorten the lifetime of the roller as well as worsen the image quality.
Furthermore, even with the foamed conductive rubber roller the toner transfer efficiency varies somewhat according to the surrounding humidity when the receiving papers are the usual papers used. This is particularly problematic for the color printing which requires a stable toner transfer efficiency, since this may cause fluctuations in colors among different printings. For this reason, it has been necessary to an set the resistivity of the roller to appropriate value which can deal effectively with the variation of the surface resistivity of the receiving papers for different humidities and different receiving papers, as described in Japanese Patent Laid Open No. S50-150437. This calls for diffusing conductive bodies of same type uniformly at constant density into the rubber, which has been extremely difficult.
In addition, when the contact pressure between the roller and the photoconductive drum is large, there appears a deterioration of the image called `middle blank` where the toner in the middle of the image is not transferred to the receiving papers. The image can also be deteriorated by the fluctuation of the image densities due to the change of the contact pressure between the roller and the photoconductive drum, caused by such things as the machine vibration. This latter becomes particularly prominent in high humidity conditions.
Moreover, it is necessary in this method to have structural complication due either to an accurate gap setting between the roller and the photoconductive drum or a pivotal configuration for a transfer roller.
The toner transfer efficiency can also be affected by the transfer bias voltage used in the electrostatic toner transfer.
Namely, for the toner transfer using the corona charger, the toner transfer efficiency increases as the transfer bias voltage is increased, but only up to some maximum toner transfer efficiency. Any further increase of the transfer bias voltage beyond this reduces the toner transfer efficiency. The best transfer bias voltage giving the maximum toner transfer efficiency tends to take higher values for more humid environment, and the maximum toner transfer efficiency tends to get lower for such case.
The present inventors has noted that this is caused by the fact that as the surrounding humidity increases the surface resistivity of the receiving papers decreases because of the moistening, which in turn causes the leakage of the corona charges, resulting in increase of the transfer bias voltages, and that as the volume resistivity decreases, the amount of inverse charges given by the receiving papers to the transferred toner increases, so that there are increased amount of the inversely transferred toner which returns to the photoconductive drum. Here, the transfer time is determined by the time taken by the receiving papers to pass through the corona charger, and this same time also gives the time for toner layer voltage, the time for the toner to transfer, and the time for the inverse charges to be given from the receiving paper to the transferred toner. This means that the toner transfer efficiency can be improved by setting an appropriate transfer time. This is also true for the transfer using the roller.
However, it has also been noted by the present inventors that, for a transfer using the roller, the toner, transfer efficiency also depends on the resistivity of the roller. Namely, for the resistivity of the roller more than 10.sup.9 .OMEGA..multidot.cm.sup.2, the toner transfer efficiency drops off as the transfer bias voltage to be applied to the toner layer on the photoconductive drum decreases, while for the resistivity of the roller less than 10.sup.7 .OMEGA..multidot.cm.sup.2, the transfer bias voltage increases too much, such that the excessive inverse charges given to the toner give rise to the increase of the inversely transferred toner.
Another problem associated with the electrophotographic printing is that a user must take the trouble of emptying an excess toner container regularly before it overfills, and refilling the emptied toner supply. One main cause for the increase of such excess toner is developing of the area on the photoconductive drum which is outside of the area to be covered by receiving papers of certain size. This ends up in wasting all of the toner on these extraneous areas, thereby increasing the amount of excess toner as well as consumed toner.
To cope with this problem, there has been made a proposition to control the corona charger so as to reduce the wasteful operation, as described in Japanese Patent Laid Open No. S56-140370. There is also another proposition to provide an additional light source for deletion of the electrostatic latent images on the photoconductive drum at the extraneous area, as described in Japanese Patent Laid Open No. S59-160159.
However, these are both attempts to control the toner electrostatically, so that they offer no solution for toner which cannot be controlled electrostatically, such as uncharged toner and toner which is physically adhered to the photoconductive drum. As a matter of fact, the amount of so called fog toner attached on the portion of the photoconductive drum without an electrostatic latent image is rather large, and rapidly increases as the photoconductive drum deteriorates. Moreover, the use of an additional light source creates various problems related to the cost, the available space, and the promotion of deterioration of the photoconductive drum due to increased illumination.
In addition, for the transfer using transfer roller, the contact between the transfer roller and the photoconductive drum with the residual toner causes the attachment of the toner onto the transfer roller, resulting in staining the backs of the receiving papers.
To cope with this problem there are propositions to separate the transfer roller from the photoconductive drum when there are no image receiving papers, as described in Japanese Patent Laid Open No. S48-40442, and to give the transfer roller a bias voltage of the same polarity as that of the toner, as described in Japanese Patent Laid Open No. S51-9840.
However, the former requires a complex mechanism for driving the transfer roller, which creates problems of size and cost, while the latter is unable to deal with those which cannot be controlled electrostatically, such as uncharged toner and toner which is physically adhered to the photoconductive drum.
As a solution to this situation, there is the proposition of a cleaning blade which wipes off the attached toner from the transfer roller, as described in Japanese Patent Laid Open No. S48-68239.
Such a cleaning blade for the transfer roller is shown in FIG. 1. The cleaning blade 301 makes a contact with the transfer roller 302 at a contact point 303 on the transfer roller 302, and good cleaning efficiency can be obtained by making an acute angle .alpha. between the cleaning blade 301 and a tangent line 304 of the transfer roller 302 at the contact point 303, and placing a support point 305 of the cleaning blade 301 before the contact point 303 with respect to a direction of rotation A of the transfer roller 301.
But, with this configuration, where the support point 305 is underneath the transfer roller 301, not only the supporting member 306 of the cleaning blade 301 gets dirty with the fall of the wiped-off toner, but also the accumulation of the fallen wiped-off toner on the supporting member 306 may interfere with the falling of the wiped-off toner itself so that the retrieval of the wiped-off toner becomes difficult.
Furthermore, this cleaning blade 301 is not effective for a soft transfer roller and causes the staining of the transfer roller 302 and the backs of the image receiving papers, as well as imperfect transfer.
Moreover, with this cleaning blade 301, a user still must take the trouble of emptying an excess toner container regularly before it overfills, which can be very frequent when the amount of the toner on the transfer roller 302 increases.
There are also other problems associated with these rollers. To put matters in perspective, it is to be noted first that the process of the electrophotographic printing essentially comprises of the following steps.
(1) the charging step in which the surface of the photoconductive drum is charged by the corona charger;
(2) the exposure step in which the surface of the photoconductive drum is exposed to light from the light source, such as a laser diode, which oscillates between On and Off states in accordance with the input signals, such that the electrostatic latent image is formed on the photoconductive drum;
(3) the developing step in which the developer such as toner is provided to visualize the electrostatic latent image on the photoconductive drum;
(4) the transfer step in which the visualized toner image is transferred onto the receiving paper;
(5) the cleaning step in which the residual image left over on the photoconductive drum after the transfer step is cleaned out; and
(6) the fixing step in which the toner image on the receiving paper is fixed by heating or other methods.
An example of a conventional laser printer performing in this manner is shown in FIG. 2.
In this laser printer, the surface of the photoconductive drum 101 is uniformly charged by the negative corona charger 102, and this surface of the photoconductive drum 101 is exposed to the scanning laser beams from the scanner 103 which oscillates between On and Off states in accordance with the input signals. The negative charges on the exposed portion of the photoconductive drum 101 are discharged and the electrostatic latent image is formed on the photoconductive drum 101. The electrostatic latent image is developed by the developing unit 104 equipped with developing roller carrying negatively charged toner. The toner image on the photoconductive drum 101 is then transferred onto the receiving paper S by positive charger 105, and the transfer sheet S is sent to the fixing unit 109 in which the toner image is fixed on the receiving paper S. Meanwhile there is some residual toner left on the photoconductive drum 101 after the transfer step. Such residual toner is cleaned by the cleaning blade 107a of the cleaning unit 107. Then, the entire photoconductive drum 101 is illuminated by the discharging lamp 106 to remove all the remaining charges, before returning to the negative corona charger 102 to repeat the process.
The excess toner collected at the cleaning step is accumulated in an excess toner container not shown, and such a user must take the trouble of emptying such an excess toner container regularly before it gets overfilled.
Also, the cleaning step is carried out by the cleaning device with the cleaning blade 107a, which is pressed against the photoconductive drum 101 to wipe along the surface of the photoconductive drum 101, which may mechanically causes damages on the photoconductive drum 101, or result in forming a film of the toner on the surface of the photoconductive drum, which can deteriorate the image quality.
One proposition to cope with this situation is to perform the developing step and the cleaning step altogether by single means, which is described in the Japanese Patent Laid Open No. S59-133573. This is based on the fact that in the electrophotographic process using reversing developing device, the charging of the photoconductive drum can be uniform regardless of the presence of the residual toner, and that with the transfer efficiency of more than 70% it is possible for the charges on the photoconductive drum to be discharged even when they are under the residual toner.
However, even in this case, some memory images appear, especially under high humidity conditions. This is due to the fact that under the high humidity conditions the transfer efficiency often drops below 70%.